Automatic instruments for measuring certain properties of yarns automatically on the run are known. However, properties such as shrinkage and crimp development in yarns which develop crimp upon heating at low tension are quite sensitive to the tension under which such crimp is developed and measured. A considerable amount of the shrinkage or crimp development in such yarns becomes evident only at very low tensions, such as are encountered in a pile carpet when the fabric is dyed at elevated temperature where the pile yarn may be under zero tension while being agitated in the dye bath. The conventional method of forming skeins and measuring their length before and after crimp development during zero-tension tumbling allows for full retraction, but this method is not continuous or automatic
Automatic instruments used for measuring shrinkage or crimp development in such yarns, such as described by M. J. Denton in U.S. Pat. No. 3,726,137 and by Lunenschloss et al. in Chemiefasern 21, 41-49 (1971) Jan., have kept the yarn under controlled low tension during heating and measuring of properties. In such cases the yarn is unable to develop the same amount of shrinkage or crimp which the yarn undergoes in normal fabric processing, and therefore the measurement does not reflect conditions of actual use. Variations in yarn properties which may cause undesirable appearance, such as configurational dye streaks in fabrics, thus may not be detected when such properties are measured with an instrument which maintains substantial tension on the yarn. This is particularly true of yarns which have a low degree of shrinkage or crimp and which are desired for high-luster fabrics.
The traditional way of beginning a measurement is to place the yarn under a standard tension using one of a variety of known tensioning devices, as described for example in U.S. Pat. Nos. 3,726,137 to Denton and 3,762,220 to Gusack. However, such tensioners are usually affected by variations in tension caused by varying drag on the yarn as it comes off a package, tension being low when the yarn is feeding from the near end of the package and higher as it feeds from the farther end. Yarn defects may cause tension plucks as yarn slides over them. Such tension variations may be magnified by conventional tensioning systems, or at least are not eliminated by them.